1. Field of the Invention
The present invention relates to a digital communication system. More particularly, it relates to a multilevel amplitude modulation and demodulation communication system including a transmitter having a modulator which modulates multilevel quadrature amplitude signals into honeycomb type multilevel amplitude signals, and a receiver having a demodulator which demodulates received honeycomb type multilevel amplitude signals into multilevel quadrature amplitude signals.
2. Description of the Related Art
In a digital communication system, particularly, in a digital radio communication system, to improve the frequency availability efficiency, a multileveling of a modulation has been found to be an effective measure, and a quadrature amplitude modulation (QAM) is a well known method of realizing such multileveling with a simple circuit configuration. In a QAM modulator, in-phase (I) and quadrature-phase (Q) data are independently converted from digital to analog, to obtain a modulation, and in a QAM demodulator, received signals are converted from analog to digital to obtain a demodulation.
To further improve the frequency availability efficiency, modulation levels of the QAM has been increased to 16, 64, and 256. However, a high modulation level requires a large transmission power, and thus a bulky transmission power amplifier having a sufficient "back-off". In addition, this large transmission power has an adverse affect on other communication systems, as a noise source. This will be described in more detail with reference to a specific example.